1. Field of the Invention
The invention relates to the field of optical measurements of turbid media and in particular, relates to an apparatus and method for the measurement of the optical properties of turbid media using frequency-domain photon migration.
2. Description of the Prior Art
The use of short light pulse propagation in multiple scattering media, such as generally described in A. Ishimaru, "Diffusion of a Pulse in Densely Distributed Scatters," J. Opt. Soc. Am. 68, 1045-50 (1978); and K. Shimizu et al., "Back Scattering of a Picosecond Pulse from Densely Distributed Scatters," Appl. Opt. 18, 3484-88 (1979) has found recent application in time domain, tissue optical spectroscopy, B. Chance et al., "Comparison of Time Resolved and Unresolved Measurements of Deoxy Hemoglobin in Brain," Proc. Nat. Acad. Sci. 85, 4971-75 (1988); and D. T. Delpy et. al., "Estimation of Optical Path Link Through Tissue from Direct Time of Flight Measurement," Phys. Med. Biol. 33, 1433-42 (1988). In contrast to previously known continuous illumination techniques such as described by R. F. Bonner et al., "Model for Photon Migration in Turbid Biological Media," J. Opt. Soc. Am. A4 423-32 (1987); and R. A. J. Groenhuis et al., "Scattering and absorption of turbid materials determined by reflection measurements. 1: Theory," Appl. Opt. 22, 2456-62 (1983), pulse propagation methods can provide information about the distribution of scatterers and absorbers in a single measurement. See M. S. Patterson et al., "Time-resolved reflectance and transmittance for the non-invasive measurement of tissue optical properties," Appl. Opt. 28, 2331-36 (1989). These optical properties may be used in a variety of therapeutic and diagnostic techniques, including imaging tissue structure, K. R. Singer et al., "Image reconstruction of the interior of bodies that diffuse radiation," Science 248, 990-93 (1990); R. L. Barbour et al., "Imaging of subsurface regions of random media by remote sensing," Proc. SPIE 1431, Los Angeles, 192-203 (1991); and D. Benaron et al., "Two-dimensional and 3-D images of thick tissue using time-contrained time-of-flight spectrophotometry," Proc. SPIE, Los Angeles, 1641 (in press), in monitoring physiology, E. M. Sevick et al., "Quantification of time and frequency-resolved optical spectra for the determination of tissue oxygenation," Anal. Biochem. 195, 330-51 (1991); B. Chance et al., "Time-resolved spectroscopy of hemoglobin and myoglobin in resting and ischemic muscle," Anal. Blochem. 174, 698 (1988); U.S. Pat. No. 4,972,331; and J. M. Schmitt et al., "Measurement of blood hematocrit by dual-wavelength near IR photoplethysmography," Proc. SPIE, 1641, Los Angeles (1992) and in predicting optical symmetry for laser based procedures, S. L. Jacques, et al., "Modeling Optical and Thermal Distributions in Tissue During Laser Irradiation," Lasers in Surgery and Medicine 6, 494-503 (1987).
The conceptional basis for using a time domain approach generally involves solutions to the radiative transfer equation described by K. Furutsu, "Diffusion equation derived from space-time transport equation," J. Opt. Soc. Am. 70, 360 (1980); and M. S. Patterson et al., "The Propagation of Optical Radiation in Tissue I. Models of Radiation Transport and their Application," Lasers in Medical Science 6, 155-68 (1991) using a Monte Carlo simulation, S. L. Jacques, "Time Resolved Propagation of Ultrashort Laser Pulses within Turbid Tissues," Appl. Opt. 28, 2223-29 (1989); and S. T. Flock et al., "Monte Carlo modeling of light propagation in scattering tissues-I. Model prediction and comparison with diffusion theory," IEEE Trans. Biotaed. Eng. 36, 1162-68 (1989) and diffusion theory approximations, K. Shimizu, supra, and A. Ishimaru, "Diffusion of Light in Turbid Materials," Appl. Opt. 28, 2210-15 (1989). Diffusion-base models provide a relatively straight forward analytical expression which describes the shape of a diffusely reflected or transmitted pulse of light in term of the optical properties of the turbid medium. See M. S. Patterson, supra, Appl. Opt. 28. Thus, the observed behavior of an ultra short light pulse can be mathematically related to the large number of optical paths available in multiple scattering media. Since introduction of losses or absorbers reduces the average path length between scattering events, the absorber-dependent changes in pulse propagation time can be used to calculate the absorption coefficients of the light in the turbid media. See Patterson, supra, Appl. Opt. 28.
Frequency domain methods can also similarly be adapted to diffusion theory models. It has been suggested in the prior art that amplitude modulated light propagates through homogeneous, multiple scattering media as diffuse waves with a coherent front. See J. Fishkin et al., "Diffusion of intensity modulated near infrared light in turbid media," Proc. SPIE, 1431, Los Angeles (1991). These photon density waves are characterized by a phase velocity, V.sub.p, and a modulation wavelength, .lambda..sub.m, that are primarily functions of the optical properties of the media. Diffuse wave properties, of course, bear no relationship to corresponding electromagnetic wave features, since in a turbid media, the phase relationships between optical waves vary in a rapid stochastic manner.
The analytical power and simplicity of frequency domain methods have been demonstrated in tissue studies up to 3 GHz, J. P. Lakowicz et al., "Frequency domain measurements of photon migration in tissue," Chemical Physics Letters 166, 246-52 (1990) and measurements of hemoglobin saturation at a single modulation frequency have also been reported, E. M. Sevick, supra. Analytic expressions have been derived for frequency domain analysis of the scattering of light in semi-infinite media from a Fourier transform of a time domain relation. See M. S. Patterson et al., "Frequency-domain reflectance for the determination scattering absorption properties of tissue," Appl. Opt. 30, 4474-76 (1991). In general, frequency-domain measurement techniques are real time recordings which, in comparison to the alternative time-domain methods, place much less stringent demands on the bandwidth of the light source and detector. Thus, the instrumentation cost can be relatively modest when laser diodes and photomultiplier tubes are employed.
What is needed is an application of the advantageous features of frequency domain methods to make practical frequency domain measurements of turbid media.